A sign which represents three, also featured it the best show ever charmed. In charmed it represnts the power of three and is featured on their book of shadows.
triquetra means many thing such as:
Christianity - father, son and holy spirit
The triple goddess - maiden, mother and crone
Charmed - the power of three
Phases of the moon - full, waxing and waning
Celtic - life, death and rebirth
Physical, mental and spiritual